Progress on the CNESM system for diagnosing high power deuterium beams of the ITER NBI prototype

The ITER neutral beam test facility under construction in Padova will host two experimental devices: SPIDER, a 100 kV negative hydrogen/deuterium RF source, and MITICA, a full scale, 1 MeV deuterium beam injector. A number of diagnostics will be deployed in the two facilities to qualify the beams [1]. This paper reports the progresses achieved on the realization of a neutron diagnostic for SPIDER, that represents a first step towards the application of this diagnostic technique to MITICA. The proposed detection system is called CNESM which stands for Close-contact Neutron Emission Surface Mapping. The CNESM diagnostic is placed right behind the SPIDER beam dump, as close to the neutron emitting surface as possible and aims at providing the map of the neutron emission on the beam dump surface. The CNESM uses nGEM as neutron detectors [2]. These are Gas Electron Multiplier equipped with a cathode that also serves as neutron-proton converter foil. The cathode is at about 30 mm from the beam dump front surface. It is designed to ensure that most of the detected neutrons at a point of the nGEM surface are emitted from the corresponding 40x22 mm2 beamlet footprint on the dump front surface. The 256 nGEM readout pads (area 12x22 mm2) will record a useful count rate of ?5 kHz providing a time resolution of better than 1 s. Each nGEM detector maps the neutron emission from a group of 5x16 beamlets: 16 nGEM would be needed to cover the entire beam dump. The CNESM was designed on the basis of simulations of the different steps from the deuteron beam interaction with the beam dump to the neutron detection in the nGEM in order to be capable to detect a 10% change in the neutron emission from individual beamlets. The main information output from the CNESM diagnostic is the map of the deuterium beamlets intensity, derived from the neutron emission map with a suitable unfolding algorithm. This information is unique and can be used to diagnose any malfunction of individual beamlets or groups of beamlets. Several small area nGEM prototypes (100 cm2) equipped with cathodes with different features have been built and tested in recent years at neutron sources (such as ISIS in the UK and FNG in Italy) in order to measure the neutron detection efficiency, directionality and gamma background levels [2-5]. Based on the small area prototype results, a first full size prototype for SPIDER with optimized geometrical parameters have been built in 2013. This detector represents the largest area neutron detector based on the GEM technology ever realized. After a first calibration using laboratory measurements, the full size prototype was successfully tested on the ISIS neutron beam where its neutron efficiency map over all the area was measured. The detector operated in the configuration that will be used in SPIDER for one week continuously without showing any issue. Further measurements will be performed in order to complete the characterization and a second prototype will be built in the next year. This work was set up in collaboration and financial support of F4E and of INFN Group 5 (Technology Research). [1] R. Pasqualotto et Al, Review of Scientific Instruments, 83, 02B103 (2012) [2] G. Croci et Al, JINST 7 C03010